Optical modulators that modulate the intensity and optical phase of a CW (Continuous Wave) light (also referred to as a continuous light below) are widely used as means for generating the optical signals used in optical transmitters. Several types of optical modulators exist. High-speed modulation of an optical signal generally uses an MZ optical modulator configured by an MZ (Mach-Zehnder) interferometer. Hereunder, in the present specification, an optical modulator refers to an MZ optical modulator unless otherwise specified. An optical modulator having a basic configuration includes an optical input terminal for inputting a CW light source, an optical output terminal for outputting a modulated signal, and a drive signal input terminal for inputting an electric data signal. In an optical modulator in an ideal state, a light intensity modulation signal or an optical phase modulation signal that correspond to an RF (Radio Frequency) drive signal input to the drive signal input terminal, is output from the optical output terminal.
In an actual optical modulator, the quality of an optical signal output from the optical output terminal may deteriorate with time due to temperature fluctuations or other reasons. For example, when an optical modulator is driven, a DC (Direct Current) bias voltage is usually applied to adjust an optical phase difference of the MZ interferometer provided in the optical modulator to an appropriate value. The optimal value of the bias voltage varies with time. This phenomenon is referred to as bias drift. In optical modulators using LiNbO3, if the bias drift is not calibrated, the optical signal deteriorates over several hours to an extent that demodulation is no longer possible. As a result, an in-service ABC (Auto Bias Control) is essential. In semiconductor optical modulators that use a change in the refractive index of the semiconductor, the bias drift is very small compared to optical modulators using LiNbO3. However, the optimal value of the bias strongly depends on the wavelength and the temperature. Consequently, automatic control of the bias is still necessary at the time of start-up of the optical transmitter and end-of-life operation, that is, during continuous operation in the warranty period.
Here, an example of a configuration in which a drive voltage and a bias voltage are applied to a semiconductor optical modulator will be described. FIG. 14 is a block diagram showing a configuration of a conventional optical transmitter. In the optical transmitter 600 shown in FIG. 14, a CS-RZ (Carrier-Suppressed Return-to-Zero) modulated light is generated in a semiconductor optical modulator 1. CW light input from the semiconductor optical modulator 1 is supplied to an MZ interferometer 2. A differential output drive amplifier 3 receives a binary data signal, and amplifies it to generate drive signals Vdata and −Vdata. The drive signals ±Vdata symmetrically distribute to positive voltage and negative voltage centering on GND (ground) level, without including a DC component. A drive signal bias voltage V4 generated by a drive signal bias voltage generator 4 is applied to the drive signals ±Vdata by a bias adder 5a and a bias adder 5b. If the modulator drive signals ±Vdata+V4 are each always positive or always negative, then the modulator drive signals are always a positive voltage or always a negative voltage, and the semiconductor optical modulator 1 is driven normally. Whether the positive voltage or the negative voltage is selected is uniquely determined by the internal structure of the semiconductor optical modulator 1. The modulator drive signals are each applied to two optical waveguides (also referred to as branches below) of the MZ interferometer 2 through a drive signal input electrode 6a and a drive signal input electrode 6b. As a result, the phases of the two lights propagating through the two branches become φ(Vdata+V4) and φ(−Vdata+V4). Here, φ(v) is a function that takes the voltage of a modulator drive signal as an argument.
Next, a moment where both Vdata and −Vdata have become the GND level is considered. In order to generate a CS-RZ light, the output light of the MZ interferometer 2 must be quenched at this moment. In other words, the MZ interferometer 2 must be biased to a null point. In order to achieve this condition, two types of voltages, namely phase difference adjustment bias voltages V70±V7, are generated by a phase difference adjustment bias voltage generator 70, and the generated voltages thereof are each applied to the two branches of the MZ interferometer 2 via phase difference adjustment bias electrodes 7a and 7b. Here, V70±V7 are set to always be positive or negative. The phase changes of the two lights propagating through the two branches caused by the phase difference adjustment bias voltages V70±V7, when expressed by a function θ(v) representing a phase difference adjustment potential bias voltage, are θ(V70+V7) and θ(V70−V7). Since “Vdata=0” at the moment both Vdata and −Vdata have become the GND level, the phase difference between the two lights propagating through the two branches is expressed by equation (1) below.{φ(V4)+θ(V70+V7)}−{φ(V4)+θ(V70−V7)}=θ(V70+V7)−θ(V70−V7)  (1)
In equation (1), nonessential terms have been omitted, and it is assumed that the characteristics of the drive signal input electrode 6a and the drive signal input electrode 6b are the same. With n as an integer, the voltage V7 is fine-tuned to satisfy equation (2) below.θ(V70+V7)−θ(V70−V7)=π×(2n+1)  (2)
Consequently, it is possible to bias the MZ interferometer 2 to a null point, and a normal CS-RZ light can be generated. In the above description, a semiconductor-type optical modulator has been described. In optical modulators using LiNbO3, either a positive or negative electric field can be applied. Further, since the drive signal bias voltage V4 and the voltage V70 in the phase difference adjustment bias voltages V70±V7 are not necessarily required, it is acceptable to set “V4=V70=0”.
Proposed as a method of adjusting the phase difference adjustment bias voltages V70±V7 to optimal values is a method that includes monitoring an optical power of a modulated light that is output from the MZ interferometer 2, and detecting a deviation from an optimal value (for example, see Non-Patent Document 1). Generally, the optical power of the modulated light that is output from the MZ interferometer 2 depends on the phase difference adjustment bias voltages V70±V7. In CS-RZ modulation, when the phase difference adjustment bias voltages are optimal, the optical power takes an extreme value, i.e., a maximum value or a minimum value. The extreme value it becomes depends on the drive amplitude, the presence of a Nyquist filter, and the like.
FIG. 15 is a graph showing a relationship of the optical power of the modulated light when the phase difference adjustment bias voltages in a conventional optical transmitter are changed from the optimal values. More specifically, FIG. 15 is a graph showing a result of simulating the relationship between the deviation from the optimal value of the bias and the optical power of the modulated light in a conventional optical transmitter. The vertical axis shows the optical power in arbitrary units (arb). The horizontal axis Vdrift shows the deviation of the bias voltage in units of Vπ@DC, that is to say, as a value normalized to Vπ@DC. Here, Vπ@DC corresponds to a half-wave voltage of the phase difference adjustment bias electrode 7a and the phase difference adjustment bias electrode 7b in FIG. 14. Circular symbols represent the case where the RMS (Root Mean Square) value of a differential voltage 2×Vdata of the drive signal is 0.8 times Vπ@RF. Square symbols represent the case where the RMS value of 2×Vdata is 0.45 times Vπ@RF. Here, Vπ@RF corresponds to a half-wave voltage of the drive signal input electrode 6a and the drive signal input electrode 6b. 
In FIG. 15, although there are some variations in the optical power among the same symbols, this is due to the calculation results obtained under different conditions being drawn superimposed. In each case, when the horizontal axis is zero, that is to say, when the phase difference adjustment bias is optimal, the optical power of the modulated light takes an extreme value i.e. a maximum value or a minimum value. This characteristic can be utilized to monitor the drift in the optimal value of the bias voltage, and it becomes possible to always maintain an optimal bias voltage.